One commonly used decontaminating agent is vaporized hydrogen peroxide. During a decontamination phase of a typical hydrogen peroxide vapor decontamination cycle, an aqueous solution of hydrogen peroxide (e.g., about 30% to 59% hydrogen peroxide, by weight) is injected into a vaporizer. The vaporizer vaporizes the aqueous solution of hydrogen peroxide, thereby generating a hydrogen peroxide vapor that is carried into an enclosure defining a region (such as a room, an isolator, a cabinet or a decontamination chamber) by a carrier gas (e.g., air). As used herein the term “decontamination” refers to the inactivation of bio-contamination, and includes, but is not limited to, sterilization and disinfection. “Decontaminant” refers to a chemical agent that effects decontamination.
Gaseous and vaporous decontamination systems rely on maintaining certain process parameters in order to achieve a target decontamination assurance level. For hydrogen peroxide vapor decontamination systems, those parameters include, but are not limited to, concentration of the hydrogen peroxide vapor, degree of saturation, temperature, pressure, and exposure time. By controlling these parameters, the desired decontamination assurance levels can be successfully obtained while avoiding condensation of the hydrogen peroxide due to vapor saturation. In this regard, condensation of hydrogen peroxide is ordinarily not desired, since it can result in increased aeration time, corrosion, and hazardous conditions. Some studies have also shown that condensation of hydrogen peroxide may also inhibit the effectiveness of the hydrogen peroxide vapor.
Considering only temperature, condensation of hydrogen peroxide occurs when the concentration of hydrogen peroxide vapor exceeds a saturation concentration (also referred to herein as a “dew point” concentration) for a given temperature. In order to avoid condensation of the hydrogen peroxide during a decontamination phase, care must be taken to insure that the actual concentration of hydrogen peroxide in the region does not exceed the saturation concentration for the temperature in the region.
Atmospheres of hydrogen peroxide vapor typically include water vapor. As the decontaminant (i.e., vaporized hydrogen peroxide) is injected into a region, the concentration of water vapor found in the region will increase due to the concentration of water in the aqueous solution of hydrogen peroxide and the degradation of vaporized hydrogen peroxide into water vapor. In addition to showing a temperature dependency, the saturation concentration of hydrogen peroxide is also a function of water vapor concentration. For example, it is observed in a vaporized hydrogen peroxide/water vapor atmosphere that the higher the actual concentration of water vapor, the lower the saturation concentration of hydrogen peroxide.
A decrease in the water vapor concentration within the region will have the beneficial effect of increasing the saturation concentration of the hydrogen peroxide. Therefore, in a typical vaporized hydrogen peroxide decontamination system, a dryer (e.g., a desiccant dryer) is used to remove moisture from the region being decontaminated.
FIG. 1 illustrates the phases of a typical vaporized hydrogen peroxide treatment cycle for a vaporized hydrogen peroxide decontaminating system. The cycle includes a drying phase, a conditioning phase, a decontamination phase and an aeration phase. The water vapor concentration and hydrogen peroxide concentration during each phase of the cycle are respectively shown by reference lines 152 and 154. During the drying phase, the region is dried to a low humidity level using a dryer (e.g., a desiccant dryer). A conditioning phase follows the completion of the drying phase. During the conditioning phase, vaporized hydrogen peroxide is injected into the region at a relatively high rate to rapidly increase the hydrogen peroxide concentration within the region. After completion of the conditioning phase, the decontamination phase commences. During the decontamination phase, injection of the vaporized hydrogen peroxide is regulated to maintain a substantially constant hydrogen peroxide concentration within the region for a required exposure time. The dryer is used during the decontamination phase to remove water vapor from the region that is produced from the breakdown of vaporized hydrogen peroxide into water vapor and oxygen. An aeration phase follows the completion of the decontamination phase. During the aeration phase, injection of vaporized hydrogen peroxide into the region is stopped and hydrogen peroxide is removed from the region until the hydrogen peroxide concentration is below an allowable threshold (e.g., 1 ppm).
Recently, there has been a need to decontaminate regions having larger volumes, such as laboratories, offices, hotel rooms, cruise ships, airport terminals, and the like. As discussed above, minimizing water vapor concentration is important to preventing condensation of hydrogen peroxide. In order to remove moisture from such large regions during the drying and decontamination phases, it may be necessary to use a large capacity dryer, multiple dryers, an on-site dryer regenerator, or a combination thereof.
With existing decontamination system control strategies, condensation poses a significant problem in the absence of dryers. In this regard, the example shown in FIG. 2 illustrates the change in the H2O2 dew point concentration (reference line 220) and water concentration (reference line 230), as the hydrogen peroxide concentration (reference line 210) is varied. At time tc, condensation of the hydrogen peroxide occurs, since the hydrogen peroxide concentration equals the H2O2 dew point concentration. The increasing water concentration continues to reduce the H2O2 dew point concentration. The parameters for the vaporized hydrogen peroxide decontamination system model associated with the data of FIG. 2 are as follows:                TREGION (temperature in the region)=25° C.        RHINITIAL (initial relative humidity in the region)=40%        VREGION (volume of region)=119 m3         Required H2O2 Concentration for Decontamination Phase=250 ppm        tEXPOSURE (Required Exposure Time)=90 minutes        Aqueous solution of hydrogen peroxide=35% H2O2/65% water, by weightIt should be understood that the system model does not account for adsorption or catalytic effects, but does account for vaporized hydrogen peroxide “half life.”        
As can be observed from FIG. 2, if water vapor is not removed from the region by use of a dryer, the water concentration in the region will increase in response to the introduction of more vaporized hydrogen peroxide and water vapor into the region. As a result, it becomes increasingly difficult to prevent condensation of the hydrogen peroxide.
The present invention provides a method and apparatus for decontaminating a region without the need for a dryer, while also preventing condensation of the hydrogen peroxide.